2976 remove useless offsetof() macros

@@ -27,10 +27,11 @@
 
 #include <sys/dklabel.h>
 #include <sys/dktp/fdisk.h>
 #include <stdlib.h>
 #include <stdarg.h>
+#include <stddef.h>
 #include <unistd.h>
 #include <strings.h>
 #include <errno.h>
 #include <limits.h>
 

@@ -145,13 +146,10 @@
 /*
  * Introduce an index into the list to speed up insert_into looking for the
  * right position in the list. This index is an AVL tree of all the
  * iodev_snapshot in the list.
  */
-
-#define offsetof(s, m)  (size_t)(&(((s *)0)->m))        /* for avl_create */
-
 static int
 avl_iodev_cmp(const void* is1, const void* is2)
 {
         int c = iodev_cmp((struct iodev_snapshot *)is1,
             (struct iodev_snapshot *)is2);